The present invention relates to a hybrid welding process and to a hybrid welding unit combining a laser beam and an electric arc, for example a plasma arc, and to their application to the welding of tailored blanks intended for the motor-vehicle industry or to the welding of pipes.
In order to lessen the drawbacks that laser welding processes or arc welding processes have, combined or hybrid processes have been developed which combine one or more laser beams with one or more electric arcs, particularly welding processes which combine a laser beam with an arc plasma jet, also called plasma/laser welding processes or more generally arc/laser processes.
Such hybrid welding processes have been described, for example in the documents EP-A-793558; EP-A-782489; EP-A-800434; U.S. Pat. Nos. 5,006,688; 5,700,989; EP-A-844042; xe2x80x9cLaser GTA Welding of aluminium alloy 5052xe2x80x9d by T.P. Diebold and C.E. Albright, 1984, pages 18-24; SU-A-1815085 and U.S. Pat. No. 4,689,466; xe2x80x9cPlasma arc augmented laser weldingxe2x80x9d by R.P. Walduck and J. Biffin, pages 172-176, 1994; or xe2x80x9cTIG or MIG arc augmented laser welding of thick mild steel platexe2x80x9d, Joining and Materials, by J. Matsuda et al., pages 31-34, 1988.
In general, arc/laser welding process are hybrid or combined welding process which combine electric arc welding with a laser beam so as to generate an electric arc between an electrode, which may or may not be consumable, and the workpiece to be welded, and to focus a powerful laser beam, especially a YAG-type or CO2-type laser, in the arc zone, that is to say near or in the joint plane obtained by joining together, edge to edge, the parts which are to be welded together.
Such a hybrid process makes it possible to considerably improve the welding speeds compared with laser welding alone or with arc welding alone. In addition, such a hybrid process makes it possible to appreciably increase the tolerances on positioning the workpieces before welding, since laser welding alone requires high precision in positioning the parts to be welded because of the small size of the focal spot of the laser beam.
The use of a plasma/laser process, and more generally an arc/laser process, requires the use of a welding head which makes it possible to combine, in a small space, the laser beam and its focusing device, and a suitable welding electrode. Several head configurations are described in the abovementioned documents and it may be stated, in summary, that the laser beam and the electric arc or plasma jet may be delivered by one and the same welding head, that is to say they leave via the same orifice, or else via two separate welding heads, one delivering the laser beam and the other the electric arc or plasma jet, their effects being combined in the welding zone.
Hybrid arc/laser processes are especially suitable for welding tailored blanks for the motor-vehicle industry, since, in addition to the abovementioned advantages, they make it possible to obtain a weld bead which is well xe2x80x9cwettedxe2x80x9d and free of undercuts, as recalled in the documents EP-A-782 489 and xe2x80x9cLaser plus arc equals powerxe2x80x9d, Industrial Laser Solutions, February 1999, pages 28-30. Such welding consists in joining together two sheets or workpieces, in general made of coated or uncoated steel, for example galvanized or zinc-plated steel, or made of an aluminium alloy, having different thicknesses. Depending on the welding methods and preparations used, the joint to be welded is conventionally characterized by a difference in level between the upper planes of each of the workpieces to be welded, thus resulting in the creation of a xe2x80x9cstepxe2x80x9d. However, the reverse situation may also be encountered, namely joints of the tailored-blank type in which the upper planes are aligned but the lower planes of which are not on the same level and in which therefore the xe2x80x9cstepxe2x80x9d is located on the reverse side of the joint to be welded.
The hybrid process is also suitable for welding many types of joints, such as, for example, an angle weld or a lap weld, or for the longitudinal or seam welding of pipes, or for the butt welding of axisymmetric workpieces, especially approximately cylindrical workpieces.
However, at the present time a problem arises, namely the fact that it has been found from the industrial standpoint that very often defects appear in the weld beads that have been produced by such a known hybrid arc/laser process, the said weld beads having an irregular and therefore unsatisfactory appearance.
Such a defect has been photographed and is shown in FIGS. 1 and 2 appended hereto, in which it may be seen that the terminal part (the region circled in FIG. 1 and enlarged in FIG. 2) of the welded joint obtained by a conventional hybrid welding process has a relatively long defect, ranging from a few centimetres to a few tens of centimetres in length. In this case, the weld beads were produced on galvanized steel sheets, held together in stainless steel devices, by a laser having a power of 3.5 kW and a 160 A/30 V plasma arc, with a welding speed of 7 m/minute.
Thus, it is an object of the present invention to solve this problem by provided an improved hybrid arc/laser welding process which allows this weld bead defect to be avoided.
The solution provided by the present invention is therefore a hybrid welding process, together with a hybrid welding unit and the use of such a process or of such a unit in the welding of workpieces for various purposes, particularly workpieces intended for the motor-vehicle industry, such as tailored blanks, or components for aircraft cells or structures, in which the dimensions require the use of welded workpieces having very different thicknesses, or else in the longitudinal welding of the two longitudinal edges of a metal strip, after they have been brought into contact with each other (forming an xe2x80x9cOxe2x80x9d) in order to make a welded pipe therefrom, the welded joint of which is oriented parallel to the longitudinal axis of the tube or is inclined with respect to the said axis in the case of the helical or spiral seam welding of a pipe.
More specifically, the invention relates to a process for welding one or more metal workpieces to be welded or assembled by using at least one laser beam and at least one electric arc to produce at least one weld bead, the said electric arc being struck between at least one electrode connected to at least a first pole of at least one current source and at least one workpiece to be welded and connected to at least a second pole of at least one current source via at least one electrically conducting earth contactor in contact with the said workpiece to be welded, characterized in that at least one contact is made between at least one earth contactor and the workpiece(s) to be welded laterally and/or upstream of the point of impingement of the electric arc on the workpiece(s) to be welded, when considering the direction of formation of the welded joint.
Within the context of the invention, the term xe2x80x9cupstreamxe2x80x9d is understood to mean a position lying on the already-welded side of the bead being produced with respect to the plane perpendicular to the welding velocity vector and passing through the point of impingement of the said arc.
Within the context of the invention, xe2x80x9cearth contactorxe2x80x9d is understood to mean a device making it possible to close the electrical circuit going from the first pole of the welding generator to the second pole of the generator, passing via the refractory electrode (in the case of plasma or TIG welding) or consumable electrode (in the case of MIG/MAG welding), the arc and the workpieces being assembled. This is therefore a device which ensures that there is electrical contact with the workpieces being assembled and which is connected via a power cable to the second pole of the welding generator(s).
According to another aspect, the process for welding at least two metal workpieces to be assembled by using at least one laser beam and at least one electric arc to produce at least one weld bead, the said electric arc being struck between at least one electrode connected to at least a first pole of at least one current source and at least one of the said workpieces to be welded and connected to at least a second pole of at least one current source via at least one electrically conducting earth contactor in contact with the said workpiece to be welded, is characterized in that at least a first contact is made between at least a first earth contactor and one of the workpieces to be welded laterally and/or upstream of the point of impingement of the electric arc on the workpieces to be welded, considering the direction of formation of the welded joint, and at least a second contact is made between at least a second earth contactor and the other workpiece to be welded laterally and/or upstream of the point of impingement of the electric arc on the workpieces to be welded, considering the direction of formation of the welded joint, the said first and second contacts being symmetrical with respect to the longitudinal axis of the weld bead produced at the junction of the said workpieces.
According to yet another aspect, the process for welding one or more metal workpieces to be welded or to be assembled by using at least one laser beam and at least one electric arc to produce at least one weld bead, the said electric arc being struck between at least one electrode connected to at least a first pole of at least one current source and at least one workpiece to be welded and connected to at least a second pole of at least one current source via at least one electrically conducting earth contactor in contact with the said workpiece to be welded, is characterized in that the shape and/or flow of the current lines are/is controlled in the workpiece(s) to be welded between the point of impingement of the electric arc on the said workpiece(s) and the said at least one electrically conducting earth contactor in contact with the said workpiece to be welded, so as to balance the forces associated with the induced electromagnetic field which act on the puddle of liquid metal obtained by melting the edges of the said workpiece(s) to be assembled and constituting, after cooling and solidifying, a welded joint, when the said current lines pass through the said puddle of liquid metal.
Depending on the case, the process of the invention may include one or more of the following characteristics:
the distance separating an earth plane passing through a point of contact between an earth contactor and a workpiece to be welded and perpendicular to the longitudinal axis of the weld bead, and the arc plane passing through the point of impingement of the electric arc on the said workpiece to be welded and perpendicular to the longitudinal axis of the weld bead, is between 0 mm and 300 mm, preferably between 1 mm and 200 mm;
two work pieces, each in contact with at least one earth contactor, are welded together, the said earth contactors being electrically isolated from one another;
contact is made between at least two earth contactors and the workpiece(s) to be welded, the said two earth contactors being placed symmetrically with respect to the longitudinal axis of the weld bead produced at the junction of the said workpieces;
the electrode is a consumable or non-consumable electrode;
the laser beam is emitted by a YAG- or CO2-type laser;
the electric arc is a plasma arc, the laser beam and the said arc preferably being delivered by a single welding head;
a gas chosen from argon, helium, nitrogen or a mixture thereof is used as assist gas for the laser beam and/or a gas chosen from argon, helium or a mixture thereof, possibly with additions of one or more secondary compounds chosen from nitrogen, hydrogen and carbon dioxide, is used as plasma gas;
during at least part of the welding stop time elapsing between the end of welding one welded joint and the start of welding the next welded joint, especially at the very start of this welding stop phase or at the end of the welding phase, the electric arc current is interrupted, attenuated or reduced, preferably the electric arc current being interrupted, attenuated or reduced approximately in intentional or delayed synchronization with the sending and/or deflection of the laser beam into the radiation absorption means allowing at least some of the radiation of the said laser beam to be absorbed.
Moreover, the invention also relates to a hybrid arc/laser welding unit for welding one or more metal workpieces to be welded or assembled, particularly by using one of the above welding processes, comprising:
at least one laser beam generator;
at least one electrode for generating at least one electric arc, especially a plasma arc;
at least one current source comprising at least a first pole to which the said electrode is connected; and
at least one electrically conducting earth contactor designed to come into contact with at least one workpiece to be welded, the said earth contactor being connected to at least a second pole of at least one current source, the point of contact between the said earth contactor and the workpiece(s) to be welded being located laterally and/or upstream of the point of impingement of the electric arc on the workpiece(s) to be welded, considering the direction of formation of the welded joint.
Depending on the case, the welding unit of the invention may include one or more of the following characteristics:
it has at least two earth contactors electrically isolated from one another, preferably the said two earth contactors being positioned symmetrically with respect to the axis of the weld bead;
it has laser radiation absorption means for absorbing, at least temporarily, at least some of the radiation of the said laser beam during the welding stop phases; workpiece support means making it possible to support and/or hold the workpieces to be welded during the welding; means for automatically feeding the workpiece(s) to be welded making it possible to feed and/or position, automatically, the workpiece(s) to be welded on the said support means; means for holding the workpieces to be welded, making it possible to keep the workpiece(s) to be welded in position on the said support means during the welding; means for automatically removing the welded workpiece(s), making it possible to remove the workpieces from the said support means after welding; and/or control means making it possible to automatically control the means for feeding the workpieces to be welded, the means for removing the welded workpieces, the means for deflecting the laser beam, the start and/or finish of the welding, and/or the relative movement of the head delivering the arc and of the head delivering the laser beam with respect to the workpieces to be welded;
it includes current balancing means making it possible to control the distribution of the electric current flowing between each earth contactor and the current source associated with the said earth contactor, preferably the said current balancing means comprising at least one electronic circuit making it possible to initiate or maintain a current balance or imbalance;
it has two earth contactors and the electrode delivering the welding electric arc, preferably a plasma arc, is supplied via one of the poles of each of the two current sources, each current source being furthermore connected via its other pole to one of the said earth contactors.
In addition, the invention relates to a process for manufacturing welded workpieces or structures intended for the motor-vehicle industry or the aeronautical industry, in which process at least one welding operation is carried out on the said workpieces by using a hybrid arc/laser welding process or a welding unit like those given above, preferably the welded workpieces or structures being vehicle body components or aircraft structures, especially tailored blanks in an end-to-end or lap configuration.
According to another aspect, the invention also relates to the use of a hybrid welding process according to the invention for assembling, by welding, metal workpieces having different thicknesses and being made of different grades and/or different metallurgical compositions.
According to yet another aspect, the invention also relates to the use of a hybrid welding process according to the invention for assembling, by welding after xe2x80x9cOxe2x80x9d forming, the two parallel longitudinal edges of a metal strip so as to manufacture a welded pipe, the axis of the weld of which is parallel or inclined to the longitudinal axis of the pipe.